Pseudo-random binary sequence generators (p.r.b.s. generators) are well known and reference should be made to the book "Cypher Systems" by Beker and Piper, 1982, published by Northwood Books, London for a description of the construction and operation of such generators. In particular, such generators can take the form of linear-feedback shift registers (LFSRs) and particular reference should be made to LFSRs of the "Galois" or "dual" kind.
P.r.b.s. generators comprise essentially an n stage recirculatory shift register and one or more associated logic gates in the loop for combining the output of at least two register stages. By appropriate choice of the logic gates a repeating sequence can be obtained of length 2.sup.n -1 bits. If n is a suitably large number this sequence is very long indeed, and the bits can be regarded as random; hence the term "pseudo random".
There may be applications where it is desired to reduce the possibilities of mimicking the generator output, and thus to increase the unpredictability of both the output signal and also the contents of the shift register stages, even when the contents of some of the shift register stages may be known.